Conventionally, this kind of multistage sprocket assembly, as disclosed in Japanese Utility Model Publication Gazette No. Sho 55-28,617 (corresponding to U.S. Pat. No. 4,268,259), includes a smaller diameter sprocket and a larger diameter sprocket assembled such that (1) the center point between a pair of adjacent teeth at the larger diameter sprocket and the center point between a pair of adjacent teeth at the smaller diameter sprocket are positioned on the tangent extending along the chain path, (2) a distance between the aforesaid center points is an integer multiple of the chain pitch, and (3) a first tooth of the larger diameter sprocket positioned behind the center point between the adjacent teeth at the larger diameter sprocket in the rotation direction for driving the bicycle is made to be easily engageable with the driving chain, thereby improving the speed change efficiency when the driving chain is shifted from the smaller diameter sprocket to the larger diameter sprocket.
The driving chain comprises a large number of pairs of inner link plates, pair of outer link plates and pins, connected in an endless manner. An interval between the opposite surfaces of each pair of inner link plates is smaller than that between the opposite surfaces of each pair of outer link plates. In other words, each pair of the outer link plates is positioned outside the inner link plates and form a space larger in width, while each pair of the inner link plate is positioned inside the outer link plates and form a space smaller in width.
The driving chain constructed as described above is biased by a derailleur toward the larger diameter sprocket so as to be shifted thereto from the smaller diameter sprocket. During shifting, when the outer link plates of the chain correspond to the first tooth of the larger diameter sprocket, since the first tooth is the easily-engageable tooth and coincides with the chain pitch, the wider space between the outer link plates is at most fitted onto the first tooth, whereby the chain engages with the larger diameter sprocket. Even when the outer link plates correspond to the first tooth as described above, the end face of a link pin and the outer surface of the outer link plate interfere with the inside surface of the larger diameter sprocket facing toward the smaller diameter sprocket side, so that the chain may not be moved further toward the outside surface of the larger diameter sprocket, such that it will not reliably engage with the first tooth.
On the other hand, when the inner link plates correspond to the first tooth of the larger diameter sprocket, the outer link plate outside the inner link plate, in turn at the larger diameter sprocket side, interferes with the inside surface of the large diameter sprocket, whereby the inner link plate does not deviate sufficiently toward the first tooth, with the result that the space between the inner link plates is not engageable with the first tooth.
In this case, when a second tooth adjacent to the first tooth and behind in the driving rotation direction of the sprocket is made easily engageable like the first tooth, a space between the pair of outer link plates behind the inner link plates in the traveling direction thereof engages with the second tooth. However, the inner link plates behind the aforesaid outer link plates may not engage with a third tooth adjacent to the second tooth and behind in the driving rotation direction, so that even when the space between the outer link plates engages well with the second tooth, the chain may fail to engage with the third tooth and ride thereon, resulting in the chain possibly disengaging from the larger diameter sprocket.
This problem can be solved by making the third tooth easily engageable like the first and second teeth. On the other hand, in a case where the outer link plates are biased in the position at which they correspond to the first tooth, the next outer link plates corresponding to the third tooth may be caught thereby. In this case, the third tooth is not positioned corresponding to an integer multiple of the chain pitch, so that it will not smoothly engage with the chain. Hence, the outer link plates caught by the third tooth may ride on the edge of the tooth crest thereof and forcibly engage therewith, thereby creating a problem in that the speed change efficiency is diminished and also in that loud sounds are generated due to the chain suddenly falling down onto the tooth bottom.